This invention relates to the production of long pulses in response to short laser pulses and in particular to the use of a fiber amplifier in a manner such that a long amplified output pulse is produced in response to a short laser pump pulse.
Pulsed output lasers, such as copper vapor lasers, can be made to have a high average power output but with very short output pulses in the order of 5-50 nanoseconds in duration. The length of the pulses is an inherent characteristic of such lasers in that a continuous population inversion necessary for lasing cannot be obtained and time is required after a lasing pulse occurs for the higher energy level to be repopulated for the next lasing pulse.
There are a variety of applications wherein it is desirable to use such lasers to produce a beam with high average power output but with "stretched" output pulses of longer length. However, such stretched pulses are often difficult to produce. For example, beam splitter arrangements have been developed using a variety of beam splitters to divide input laser pulses and direct the divided pulses to different paths having mirrors arranged to provide different length optical delay paths for the pulses to traverse. The differently delayed pulses are then recombined into a single output pulse of a longer duration. The disadvantage of such a system is that it requires a very careful and precise alignment of the various beam splitters, mirrors and recombiners to produce a useful output.
Heretofore unrelated to the area of pulse stretching are fiber amplifiers. Laser amplification in a fiber medium is becoming a common practice, especially in communications wherein modulated light signal beams passing through a fiber optic system are periodically amplified to compensate for the attenuation of the beam as it travels through the system. Such amplifiers use an optical amplifier fiber with a suitably doped inner core of material which will provide an amplifying medium for light of the color of the signal beam when the core is driven into an excited state by light of a shorter wave-length. Typically, the amplifiers include a laser with a continuous output at the shorter wavelength positioned adjacent the doped core to excite a fixed region of the core and amplify the signal beam passing through that region.